The concept of diligence goes back to the Roman law concept of diligentia, distinguishing two main types of diligence: diligentia quam suis rebus, or the care that an ordinary person exercises in managing his or her own affairs, and diligentia exactissima or diligentia boni patrisfamilia, a more exacting type of care exercised by the head of a family.
‘Reasonable man test’ given in Blyth v Birmingham Waterworks (1856) 11 Ex R781 by Alderson B at p.784:
Negligence is the omission to do something which a reasonable man, guided upon those considerations which ordinarily regulate the conduct of human affairs, would do, or doing something which a prudent and reasonable man would not do. The defendants might have been liable for negligence, if, unintentionally, they omitted to do that which a reasonable person would have done, or did that which a person taking reasonable precautions would not have done.
Scriven Brothers & Co v Hindley & Co  3 KB 564, per A. T. Lawrence J:
A buyer when he examines a sample does so for his own benefit and not in the discharge of any duty to the seller; the use of the word 'negligence' in such a connection is entirely misplaced, it should be reserved for cases of want of due care where some duty is owed by one person to another.
Blue Anchor Line Ltd. v Alfred C. Toepfer International (The Union Amsterdam)  2 Lloyd’s Rep. 432 per Parker J at p.435:
I should add, however, that whereas here there was no submission to the arbitrator that there was no evidence which could in law justify a finding of negligence, a party who seeks to challenge a finding that there was can very rarely succeed.